Interpretive Summary: Antimicrobial resistance (AR) in bacteria is a major concern for human and animal health. Infections caused by organisms carrying AR and multi-drug resistance (MDR) can be difficult to treat and can lead to serious complications. The prevalence of AR in bacteria isolated from food animals has increased recently as have concerns of foodborne infections in humans by bacteria with AR. To understand this, investigators have monitored AR and are determining the genetic mechanisms leading to resistance among bacteria in food animals. MDR mechanisms in Salmonella enterica isolated from U.S. animals are mostly located on self-replicating extra chromosomal genetic elements called plasmids, and integrons, which are genetic elements that can be integrated into the chromosome or into plasmids. These genetic elements are dominated by the MDR IncA/C plasmids, other plasmids, and MDR integrons all of which are mobile and can be easily transferred among bacteria. These elements may have their origins in environmental or nonpathogenic commensal bacteria. Some of the resistant Salmonella isolated from humans in the U.S. have different mechanisms of resistance, suggesting the possibility of a non-food source for some human MDR infections. Commensal bacteria including Escherichia coli and Enterococcus spp. in animals may serve as reservoirs for resistance mechanisms and have also been investigated. Some of the resistance and mobile genetic elements found in E. coli isolated from U.S. animals are very similar to those found in Salmonella, including MDR IncA/C and other resistance plasmids. Enterococcus spp. isolated from animals frequently carries AR including resistance to macrolides, glycopeptides, and tetracyclines. To ensure the utility of antimicrobials, continuing investigations are required to help us understand the mechanisms of resistance so that control measures can be developed to protect human and animal health.

Technical Abstract:
The development and spread of antimicrobial resistance (AR) in bacteria are a global concern for humans and animals. Infections by organisms carrying AR and multi-drug resistance (MDR) can be difficult to treat and lead to serious complications. Since the mid 1990’s, the prevalence of AR in bacteria isolated from food animals and the animal environment has increased as have concerns of foodborne zoonotic infections in humans by bacteria exhibiting AR. To understand the factors driving this phenomenon, investigators have monitored AR and studied the genetic mechanisms leading to resistance and its spread among bacteria in food animals. Mechanisms leading to MDR in Salmonella enterica animal isolates mostly involve antimicrobial resistance genes located on plasmids and integrons. These genetic elements are dominated by the MDR IncA/C plasmids, other plasmids, and MDR integrons, which be easily transferred among Salmonella and other bacteria. These elements may have their origins in environmental or commensal bacteria. Although it is generally thought that antimicrobial resistance in enteric pathogens develops in food animals and is then transmitted to humans, some of the resistant Salmonella isolated from humans in the U.S. have different mechanisms of resistance, suggesting the possibility of a different etiology for some human MDR infections. The resistance mechanisms and genetic elements identified in animal isolates from outside the U.S. are also predominantly different from those found in the U.S. Commensal bacteria including Escherichia coli and Enterococcus spp. in animals may serve as reservoirs for resistance mechanisms and have also been investigated in depth. Some of the resistance and mobile genetic elements found in E. coli isolated from U.S. animals are very similar to those found in Salmonella, including MDR IncA/C and other resistance plasmids. Enterococcus spp. isolated from animals frequently carries AR including resistance to macrolides, glycopeptides, and tetracyclines, conferred by genes located on mobile elements such as plasmids and transposons. To ensure the utility of antimicrobials, continuing investigations are required to help us understand the impact of AR bacteria on human and animal health.